NATO chief Jens Stoltenberg says that the alliance must counter a Russian military buildup from the Baltic to the eastern Mediterranean that could enable Moscow to control key areas in a crisis.

Speaking at a news conference in Portugal during NATO war games, he said the 28-nation alliance must also consider doing more to reassure eastern member states once allied with Moscow who have been unnerved by Russia's intervention in Ukraine.

As ties with the West have deteriorated, Russia has boosted its military presence in its Kaliningrad exclave, which sits west of the Baltic states.

Moscow last year annexed Crimea and bolstered its presence in the Black Sea region, while this year it has deployed troops, aircraft, and navy ships to Syrian ports on the Mediterranean to prop up longtime ally President Bashar al-Assad.

Stoltenberg warned that Russia was acquiring the ability and presence to exercise control over strategic points and NATO must ensure it can carry out its own missions in such a changed environment.

"This is a military buildup which provides the Russians with what many experts call Anti-Access/Area Denial (A2/AD) capabilities," he said.

"We have to be sure we are able to overcome these capabilities so we can reinforce and deploy forces if needed," he said, after watching troops take part in the Trident Juncture exercise, NATO's biggest in more than a decade.

"The question on our agenda now is how to overcome, how to deal with the increased A2/AD capabilities of Russia in the Baltic, the Black Sea, and now in the Mediterranean."

Stoltenberg's remarks came in response to a question about whether NATO should do more to reassure eastern states which have grown fearful in the face of a more assertive Russia.

They want NATO to do more, and this week proposed that the alliance even set up permanent bases on their soil.

NATO has previously ruled out that possibility for fear of breaching a 1997 treaty with Russia banning such a presence.

But Stoltenberg said that "the important thing is that we have military presence" there, and he saw no real distinction between permanent bases and the temporary, rotating NATO deployments of troops, ships, and planes that have all been stepped up since the Ukraine crisis broke out.

NATO has also set up forward command units in Estonia, Latvia, Lithuania, Hungary, Bulgaria, and Romania, and prepositioned equipment so that its new, high-speed rapid-reaction force can hit the ground running in any crisis.

"We have already increased our presence and we are looking into the question of whether we should increase it even more," Stoltenberg said, adding that the issue would be on the agenda for the next NATO leaders summit in Warsaw in July 2016.

Stung by Russia's intervention in Ukraine, NATO leaders agreed last year to reverse years of defense-spending cuts and to upgrade their rapid-response force, more than doubling its size to around 40,000 troops.

They also approved the Very High-Readiness Joint Task Force, a smaller "spearhead" unit that can put boots on the ground within 48 hours.

The Trident Juncture exercise is NATO's biggest since 2002, putting some 36,000 troops through their paces over five weeks in Italy, Spain, and Portugal.